1. Field of the Invention
The present invention relates to tools for making electrical contact elements for electrical devices, and more particularly to forming tools for forming moldable substrates for making lithographic-scale, microelectronic spring contacts.
2. Description of Related Art
Molding, embossing, and like forming techniques have not previously been required or used in the field of semiconductor device manufacture, microelectronic spring contact manufacture, or any related area of micro-electrical-mechanical (xe2x80x9cMEMxe2x80x9d) device manufacturing. However, new methods for forming microelectronic spring structures, as described for the first time in the parent applications referenced above, utilize a mold for a microelectronic spring structure which is contoured in a direction perpendicular to the underlying substrate. In many embodiments, the mold is formed by impressing a layer of sacrificial material with a forming tool. These new methods are well suited for making numerous microelectronic, very fine pitch spring structures in parallel, lithographic mass production processes that are readily repeatable and reproducible. After the formable sacrificial substrate is formed into a suitable mold, resilient spring material is deposited in the mold and patterned. Thus, the new methods are capable of making spring structures having various defined three-dimensional contoured shapes for providing improved spring characteristics, such as improved strength, stiffness, resistance to stress concentration cracking, and elastic range. However, to efficiently form the sacrificial substrate into the desired shape, a suitable forming tool (sometimes called a stamping tool) is needed. Given the complete lack of such tools in any related field of art, suitable tools are not currently known, nor are methods for making suitable tools. A need therefore exists for a forming tool for forming a sacrificial substrate used in the manufacture of microelectronic springs, and for a method of making such a forming tool.
The present invention provides a forming tool suitable for forming a sacrificial substrate, which may in turn be used for forming microelectronic spring structures. The present invention further provides methods for making such a forming tool.
A forming tool according the present invention is provided with one or more embossing tooth, and preferably, a plurality of such embossing teeth, arranged on a substantially planar substrate. Each embossing tooth is configured for forming a sacrificial layer to provide a contoured surface in the shape of a desired spring structure. Spring structures of the defined contoured shape may then be formed by deposition of a suitable resilient material on a sacrificial layer that has been embossed using the forming tool. Thus, a sacrificial layer may be prepared for formation of a plurality of spring structures in parallel using a forming tool with a plurality of embossing teeth.
The forming tools with contoured embossing teeth according to the present invention are suitable for forming sacrificial substrates for making springs with numerous performance improvements. For example, forming tools according to the present invention may be used to readily form spring structures having a U-shaped cross-section, a V-shaped cross-section, and/or a rib running along a length of the spring. These contoured shapes, and the advantages provided thereby, are described in the co-pending application Ser. No. 09/710,539, referenced above.
Various methods for making a forming tool according to the present invention are provided. In one embodiment, a relatively hard substrate is selected and the embossing teeth are defined by selective removal of material from the substrate. Various methods may be used to selectively remove material, including laser ablation, mechanical milling using a precision diamond saw, and gray-scale lithography using photo-patternable glass. The hard substrate with embossing teeth may be used directly as an embossing tool.
Alternatively, the hard substrate may be used to form a reusable mold for producing a plurality of identical forming tools, thereby reducing costs associated with selective material removal. In this embodiment, the hard substrate is used to form shaped impressions corresponding to the embossing teeth in a moldable layer. A layer of tool material is then deposited in the molded layer. The tool material may be a relatively hard material such as a nickel or nickel alloy, a relatively soft material such as an organic polymer, or a UV transparent material such as SU-8. The molded layer is then removed from the layer of tool material to produce the forming tool. In some embodiments, the molded layer may be reused to mold another forming tool. Alternatively, the molded layer is destroyed after each use.
In another embodiment, a method for making a forming tool using the properties of a liquid meniscus is provided. First, a layer of material is formed over a substrate. Then, a recess is developed in the material, and liquid is provided in the recess to form a meniscus. The liquid is cured or hardened to stabilize the contoured shape of the meniscus. The stabilized meniscus is then used to mold an embossing tooth of a forming tool, as described above. A spring structure deposited on a sacrificial substrate that has been formed with the embossing tooth will take on the shape of the liquid meniscus.
In another embodiment of a method for making a forming tool, a thin metal sheet is etched to define a plurality of cantilevered metal tangs having the projected shape of the desired spring structures in plan view. The sheet is adhered to a planar substrate and a free tip of each tang is uniformly deflected away from the substrate using a precisely positioned spacer. A stabilizing fill material is then used to hold the thin sheet in place with the tangs in their deflected position, and the substrate is removed. The fill material is selectively removed to define embossing teeth for a forming tool. Removal of fill material underneath the tangs yields a stabilized sheet that may be used directly as a forming tool, or as a plug for making a forming tool mold. Alternatively, removal of fill material above the tangs yields a mold that may be used to form a forming tool as described above.
In another embodiment of a method for making a forming tool, thin metal tangs of bimetallic or other pre-stressed material are deposited on a substrate using a lithographic process, in the desired spring shape. Each tang is adhered to the substrate at one end. Application of heat or other method is used to create a stress gradient across the thickness of the tang, causing the tangs to bend away from the substrate at their free ends, defining contoured spring shapes. A stabilizing fill material is applied to stabilize the tangs in their contoured shape. The fill material is selectively removed to define a forming tool, or a mold for a forming tool, as previously described.
In another embodiment of a method for making a forming tool, an embossing tooth is fabricated from a UV transparent material, preferably by a lithographic process. The tooth is freed from the surrounding material and attached to a UV transparent material, such as the end of a light guide. The tooth is then used to mold contoured surfaces in a layer of UV curable material on a substrate. After each contoured surface is molded, it is cured by exposure to UV light. After a desired number of contoured surfaces have molded and cured, the uncured portions of the layer of UV curable material are removed from the substrate. The resulting substrate with cured contoured surfaces is then used to mold a forming tool, as previously described.
In another embodiment of a method for making a forming tool, thin plates or layers of material are patterned in the shape of cross-sections of a forming tool. The material may be a metal, a UV transparent material such as SU-8, or any other patternable material of sufficient strength. The patterning may be done by a precision machining method such as laser ablation, or by a photo-lithographic and etching process. The layers or plates are stacked in the desired sequence and fastened together to form a section of a forming tool. Additional sections may similarly be assembled together to form a forming tool, or plug mold for making a forming tool, as previously described.
A more complete understanding of the forming tool and method for making the same will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.